Final answer:
Vestigial structures are anatomical features that have lost their original function through evolution, indicating a common ancestry with organisms that once used those structures.
Step-by-step explanation:
Anatomical structures that appear to be derived from a functional structure in an ancestor, but that currently do not serve an important function, are called vestigial structures. Examples of vestigial structures include the wings on flightless birds like the ostrich, leaves on some cacti, and the hind leg bones in whales. These structures provide important evidence of evolution because they indicate that the organism has changed from one that once used the structure to one that no longer uses it, or uses it for a different purpose. The presence of vestigial structures in an organism suggests that it shares a common ancestry with other organisms that have or had similar structures.